Lightweight metal foams have high strength-to-weight ratios and are extremely useful as load-bearing materials and as thermal insulators. Metal foams are characterized by high impact energy absorption capacity, low thermal conductivity, good electrical conductivity and high absorptive acoustic properties.
A particle stabilized metal foam of exceptional stability is described in Jin et al U.S. Pat. No. 4,973,358, issued Nov. 27, 1990. According to that patent, a composite of a metal matrix and finely divided solid stabilizer particles is heated above the liquidus temperature of the metal matrix. Gas is then introduced into the molten metal composite below the surface of the composite to form bubbles therein. These bubbles float to the top surface of the composite to produce on the surface a closed cell foam. The foam which forms on the surface of the molten metal composite is a highly stable liquid foam, i.e. the foam cells do not collapse under their own weight. This stable liquid foam is then cooled below the liquidus temperature of the melt to form a metal foam product having a plurality of closed cells and the stabilizer particles dispersed within the metal matrix.
A method for shaping metal foam is described in Niebylski et al, U.S. Pat. No. 3,873,392, issued Mar. 25, 1975, in which solid metal foam is compressed such that cell walls are crushed. Although heat may be used, it is preferred that the temperature does not exceed about 38.degree. C. below the melting point of the base metal.
Another method for shaping a metal foam body is described in Erb, U.S. Pat. No. 3,595,059, issued Jul. 27, 1971. In this method, the forming device is reciprocated causing localized heating and crushing of the walls of the foam structure.
Shape casting of molten metals, such as aluminum, can be carried out in a wide variety of closed moulds. One such technique is squeeze casting, also known as liquid-metal forging, in which molten metal solidifies under pressure within closed dies positioned between the plates of a hydraulic press. The applied pressure and the instant contact of the molten metal with the die surface produces a rapid heat transfer condition that yields a pore-free fine-grain casting with mechanical properties approaching those of a wrought product. Semi-solid metal working is also used, which incorporates elements of both casting and forging. This may be referred to as rheocasting, thixocasting or stir casting. In this procedure a thixotropic material is formed which can be moved and handled.
It is the object of the present invention to provide a shape casting technique for particle stabilized metal foam which takes advantage of the unique characteristics of the particle stabilized metal foam.